1. Field of the Invention
The present invention relates to a magnetic recording medium such as an audio and a video tape, a floppy diskette, and a hard disk. In particular, the present invention relates to a technique for coating magnetic thin films to provide magnetic recording media having improved properties.
2. Prior Art
A magnetic recording medium such as an audio and a video tape, a floppy diskette, and a hard disk is generally constructed by forming a magnetic film on a substrate support made of an organic resin or a metal and the like. The magnetic film is formed by coating or by a vacuum process such as vapor deposition and sputtering. If the process cost is the matter of concern, coating is the choice in forming the magnetic film, and if high performance is required on the obtained film, a vacuum process is selected.
When in use, magnetic recording media are under a mechanical load because they are constantly or temporarily brought into contact with a magnetic head (an apparatus for writing and reading signals) or a mechanical component thereof, and with mechanical parts such as a capstan and a roller. Accordingly, not only magnetic properties but also mechanical durability and lubricity are required to the magnetic recording media to resist the above mechanical load. Conventionally, attempts had been made to lower the friction coefficient of the magnetic media, for example, by applying a lubricant coating to the surface of the magnetic film, or by forming irregularities on the surface to substantially reduce the contact area between the contact member and the magnetic film. However, the lubricant film suffers wear by repeated contact and sliding, to result in an increase of the friction coefficient thereof. Accordingly, the degradation proceeds in an accelerated manner.
Recent trend treats information which had been conventionally analog signals such as images and sound as digital signals. Accordingly, the reliability of magnetic recording medium on recording is required. Under such circumstances, the friction coefficient of a magnetic recording medium must be maintained stably at a low level.
A magnetic thin film is formed by either coating a substrate with a mixture of a powder of a magnetic material and an organic polymer, or by forming a layer of the magnetic material alone on a substrate by a vacuum process such as vapor deposition and sputtering. Thin films having better magnetic properties can be obtained by a vacuum process. The magnetic material is generally a ferrite or a metal, etc. Metallic magnetic materials specifically include NiCo and CoPtCr alloys.
Alternatively, methods for forming a hard coating of carbon or a material containing carbon as the principal component by plasma CVD process are disclosed, for example, in JP-B-3-72711, JP-B-4-27690, and JP-B-4-27691 (the term "JP-B-" as referred herein signifies an "examined Japanese patent publication"). Those well known processes comprise introducing a hydrocarbon gas and a hydrogen gas as material gases inside a vessel maintained under a reduced pressure, applying generally a high frequency electric field to a pair or more electrodes installed inside the vessel, and forming a plasma of the material gas to thereby activate and deposit grains containing carbon on the substrate.
The coating thus obtained is called diamond-like carbon (abbreviated as DLC hereinafter) because it is extremely hard and exhibits diamond-like characteristics. In general, DLC is deposited by applying self bias or a bias from an external power supply, so that a negative bias with respect to the plasma potential may be applied to the substrate. In this manner, the bonds having the graphite-like characteristics (attributed to the combination of sp.sup.2 hybrid orbital and p orbital) within the carbon film are etched to leave mainly the bonds exhibiting the diamond-like characteristics attributed to sp.sup.3 hybrid orbital.
Such a DLC thin film is very hard as to yield a Vicker's hardness of 2,000 kg/mm.sup.2 or even higher, and is also low in friction coefficient. Accordingly, the DLC film is suited as a protective and lubricant film to coat the surface of a magnetic film.
When applied to the surface of a magnetic material, and particularly to the surface of a metallic magnetic material, however, the aforementioned DLC film easily undergoes separation after film deposition, or suffers peeling off upon bringing it in contact with or sliding it against a mechanical component. The DLC films were not practically feasible, therefore, due to their insufficiently low adhesion strength.